This invention relates to radiation of RF energy from a surface wave transmission line. More specifically, this invention relates to an RF transmission and radiation system in which a reflector that is connected to the end of the surface wave transmission line exhibits dual polarization over a broadband of frequencies.
As is known in the art, RF electromagnetic energy will propagate along a single conductor that is configured or treated to concentrate and confine the electromagnetic energy to a cylindrical volume that coaxially surrounds the conductor. This type of transmission line is known as a surface wave transmission line, a Goubau line or a G-line. In the more commonly known surface wave transmission lines, a conductor is surrounded by a coating of low-loss dielectric. Since the phase velocity of the electromagnetic energy that propagates through the dielectric coating is less than the free space phase velocity of the propagating signal, substantially all of the electromagnetic energy is confined to the dielectric and a cylindrical volume of space that concentrically surrounds the dielectric coating. Other techniques for suitably decreasing the phase velocity of the propagating signal also are known. For example, crimping an uncoated wire or machining thread-like grooves in the wire surface will cause a reduction in phase velocity in signals traveling along the wire, thereby causing the uncoated wire to act as a surface wave transmission line.
Since surface wave transmission lines provide a highly efficient transmission medium (low-loss operation) and will support electromagnetic wave propagation over a wide frequency range (broadband operation), application is found in various situations in which an environmental situation can accommodate the unique properties of a traveling surface wave. One such application is a surface wave transmission and radiation system wherein a wire (surface wave transmission line) is towed by an aircraft and one or more radiators that are located at or near the distal end of the wire cause the propagating surface wave energy to be detached from the wire (i.e., radiated into space). Examples of this type of aircraft-surface wave transmission and radiation system are disclosed in copending U.S. Pat. No. 4,743,916, Ser. No. 813,049, filed Dec. 24, 1985, by G. A. Bengeult and entitled "Method and Apparatus For Proportional RF Radiation From Surface Wave Transmission Line"; and in my copending U.S. patent application, which is being filed on even date with this patent application, and is entitled "Apparatus For Circularly Polarized Radiation From Surface Wave Transmission Line." In the systems disclosed in both of the referenced patent applications, an electromagnetic wave that is to propagate along the surface wave transmission line is coupled to the transmission line by a rearwardly facing horn-like surface wave "launcher." The launcher, in effect, serves as a transition between the surface wave transmission line and a coaxial cable or waveguide that serves as a feed line that interconnects the surface wave transmission line with the aircraft RF transmitter or transceiver.
In the radiation system disclosed in the referenced patent application by Bengeult, a series of two or more electrically conductive radiating elements that are spaced apart by a distance greater than one wavelength (relative to the RF electromagnetic energy that propagates along the surface wave transmission line) are configured in a manner that causes radiation of the RF electromagnetic energy that impinges on the radiator or radiators. When viewed from the far field, the result is that each radiator appears to be a horizontally polarized separate source of radiation.
In my referenced patent application, an arrangement of two radiators that are spaced apart from one another are configured and arranged to produce circularly polarized radiation. To attain this result, the forwardmost radiator includes an annular conductive region that surrounds the surface wave transmission line with a pair of spiral antenna arms extending outwardly along the surface of the radiator from oppositely disposed positions on the outer boundary of the annular conductive region. The second radiator, which is located at the terminus of the surface wave transmission line, includes a circular conductive region to which the surface wave transmission line is connected and further includes a pair of spiral antenna arms that extend outwardly along the surface of the radiator. The annular opening in the forwardmost radiator is dimensioned so that one-half of the incident surface wave energy is radiated by the forwardmost radiator and the remaining one-half of the electromagnetic energy propagates through the circular opening of the annular conductive region and is radiated by the second radiator. The orientation between the forwardmost and second radiators is established both with respect to axial distance between the radiators and the spatial position of the inner ends of the spiral antenna arms to cause the individual signals radiated by the two radiators to combine in a manner that results in the desired far field circular polarization.
Because the system disclosed in my referenced patent application provides circularly polarized electromagnetic radiation, that system may be advantageously employed in situations in which the radiated electromagnetic energy that is to be received by one or more antenna arrangements are of unknown polarization. Even though such a system provides precise and uniform circularly polarized radiation that will yield near optimal performance with receiving antennas of any polarization, a need exists for other surface wave radiation devices that produce electromagnetic energy that is polarized in more than one direction. Specifically, in some situations, the radiated electromagnetic energy may be received by antennas that exhibit either vertical polarization or horizontal polarization, but do not exhibit polarization that is angularly oriented with respect to the vertical or horizontal axes. In such a situation, dual polarization of the transmitted electromagnetic energy (vertical and horizontal) will provide substantially the same result as circularly polarized radiation. In addition, dual polarized radiation will often suffice even through the receiving antenna may be polarized in any direction. That is, although receiving antennas that are not horizontally or vertically polarized will not generate as great a signal when a dual polarized transmitting arrangement is utilized, the off-axes loss of signal may not seriously affect overall system operation. The acceptance of dual polarized radiation instead of circularly polarized radiation can be further enhanced in the event that the radiating apparatus is simpler in structure and more economical to manufacture than the apparatus that provides the circularly polarized electromagnetic radiation.
Accordingly, it is an object of this invention to provide an arrangement for radiating dual polarized electromagnetic energy from the terminus of a surface wave transmission line.
It is a further object of this invention to achieve dual polarized radiation in a surface wave transmission and radiation system in a manner that lends itself to easy and economical fabrication.